1. Field of the Invention
This invention relates to a process for preparing a magnetic fluid, and more particularly to a process for preparing a magnetic fluid having an improved saturation magnetization by stably dispersing fine particles of ferrite in a low vapor pressure base oil at a high concentration.
2. Description of the Prior Art
Fine particles of ferrite are prepared by pulverization, co-precipitation, vapor deposition, etc., although a co-precipitation procedure is usually used from the viewpoint of purity, particle size control, productivity, etc. However, co-precipitation is a precipitation reaction from an aqueous solution containing iron ions, and thus the resulting fine magnetic particles are in a suspended state in an aqueous solution.
On the other hand, it is desirable that the fine particles of a magnetic material for a magnetic fluid are not in a coagulated state, but in an individually dispersed state. Thus, in the case of fine particles of a magnetic material prepared by co-precipitation it is necessary to adsorb a surfactant so as to prevent a coagulation or agglomeration onto the surfaces of the fine magnetic particles in a dispersed liquid state without passing through a drying step which causes a risk of coagulation and agglomeration of the fine particles themselves. Therefore, a water-soluble surfactant is used.
A dispersing base oil for a magnetic fluid containing dispersed fine particles of water-soluble surfactant-adsorbed magnetic material is restricted to a relatively volatile solvent such as kerosine, toluene, etc., and when the magnetic fluid is used as a magnetic fluid seal, or for magnetic fluid polishing, etc., evaporation of the base oil is a problem so important as to deteriorate the function of the magnetic fluid itself.
Generally, a magnetic fluid is a dispersion of fine particles of ferrite in a base oil by using a dispersing agent such as a higher fatty acid salt, sorbitan ester, etc. However, any higher dispersibility cannot be obtained from a mere dispersion of fine particles of ferrite in a low vapor pressure base oil and such a dispersion is not used in practical applications.
Even if a higher dispersibility could be obtained from such dispersion in a low vapor pressure base oil, it would take much time to form a uniform suspension, because the low vapor presure base oil has a high kinematic viscosity such as 8-50 Cst (40.degree.) in contrast to those of ordinary organic solvents or water such as 1 Cst or less (40.degree.). Still furthermore, all of the fine particles of ferrite to be dispersed are not formed into a stable suspension, and a considerable proportion of the fine particles of ferrite is removed at its purification by centrifuging, etc. There is also a problem of a very low efficiency.
Prior art of using a low vapor pressure base oil as a dispersing base oil for a magnetic fluid containing fine particles of water-soluble surfactant-adsorbed magnetic material in a dispersed state is disclosed in Japanese Patent Application Kokai (Laid-open) No. 58-174,494, U.S. Pat. No. 4,430,239, etc.
Japanese Patent Application Kokai (Laid-open) No. 58-174,494 discloses a process for preparing a magnetic fluid by adding a water-soluble surfactant and a low boiling point organic solvent to fine particles of a magnetic material, thereby forming a suspension of fine particles of surfactant-adsorbed magnetic material dispersed in the low boiling point organic solvent, then adding a low vapor pressure base oil and a polyether-type, nonionic surfactant as a dispersing agent to the suspension, removing then low boiling point organic solvent therefrom by distillation, and dispersing the fine particles of the magnetic material in the low vapor pressure base oil.
U.S. Pat. No. 4,430,239 discloses a process for preparing a magnetic fluid by using a long chain alcohol ester of phosphoric acid as a dispersing agent in place of the polyether-type, nonionic surfactant in the process of said Japanese Patent Application Kokai (Laid-open) No. 58-174,494.